A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive traces, pads and other features etched from electrically conductive sheets, such as copper sheets, laminated onto a non-conductive substrate. Multi-layered printed circuit boards are formed by stacking and laminating multiple such etched conductive sheet/non-conductive substrate laminations. Conductors on different layers can be interconnected with plated-through holes called vias.
FIG. 1 illustrates a cut out side view of a portion of a conventional PCB. The PCB 2 includes a plurality of stacked layers, the layers made of non-conductive layers 4, 6 and conductive layers 8. The non-conductive layers can be made of prepreg or base material that is part of a core structure, or simply core. Prepreg is a fibrous reinforcement material impregnated or coated with a thermosetting resin binder, and consolidated and cured to an intermediate stage semi-solid product. Prepreg is used as an adhesive layer to bond discrete layers of multilayer PCB construction, where a multilayer PCB consists of alternative layers of conductors and base materials bonded together, including at least one internal conductive layer. A base material is an organic or inorganic material used to support a pattern of conductor material. A core is a metal clad base material where the base material has integral metal conductor material patterned on one or both sides. A laminated stack is formed by stacking multiple core structures with intervening prepreg and then laminating the stack. A via 10 is then formed by drilling a hole through the laminated stack and plating the wall of the hole with electrically conductive material, such as copper. The resulting plating 12 interconnects the conductive layers 8. In the exemplary application shown in FIG. 1, the plating 12 extends uninterrupted through the entire thickness of the via 10, thereby providing a common interconnection with each and every conductive layer 8. In other applications, one or some of the conductive layers do not extend laterally to the via and therefore are not connected to the plating within the via for interconnection with other conductive layers. In general, a conductive layer can represent a patterned interconnect layer, a ground plane or a power plane.
A significant consideration when designing a PCB and PCB assembly is a thermal solution for dissipating heat generated by such devices. Many electronic products need to consider the thermal solution in order to make the system work properly. In many applications, the thermal problem becomes particularly acute, such as for high power systems and related IC packaging. In such applications, a heat sink is often attached on the back side of the PCB. Heat generated by electronic components mounted to the front side of the PCB can be transferred to the heat sink using copper plated vias or copper coins included within the PCB. FIG. 2 illustrates a cut out side view of a portion of a conventional PCB with a heat sink attached. For simplicity, the individual stacked layers of the PCB are not shown. The PCB 18 includes a plated through via 20 with via side wall plating 22 and an annular ring 24 on both the front side and the back side of the PCB 18. Soldermask 26 is also shown in the front side of the PCB 18. A copper heat sink 30 is attached to the backside of the PCB 18 by thermally conductive adhesive 28. Heat generated on the front side of the PCB 18, such as by attached electronic components (not shown) is transferred through the via 20 by the via side wall plating 22 to the annular ring 24 on the back side of the PCB 18 and to the thermally attached heat sink 30. The via 20 functions in this case as a thermal via, not as a signal carrying via, or simply signal via, as with the signal via 10 shown in FIG. 1. A via functioning as a signal via cannot be connected to the heat sink because the heat sink is also electrically conductive and would short circuit the signal via.
However, as circuit densities increase, the area of the PCB under the heat sink may also be need for signal vias. In this case, such signal vias need to be designed such that there is no conductive contact between the signal via at the backside of the PCB and the heat sink. This can be accomplished by either removing the portion of the heat sink that is positioned over the signal via or by removing the via plating at the backside of the PCB. To remove the via plating, a back drill process is performed where a hole is drilled into the back side of the PCB at the signal via. The drilled hole diameter is wider than the via diameter such that the drilled hole removes a portion of the via side wall plating in addition to at least a portion of the via back side surface annular ring. FIG. 3 illustrates a cut out side view of a portion of a conventional PCB having the via back drilled. The PCB 42 is similar to the printed circuit board 18 of FIG. 2 except that a hole 40 has been back drilled into the PCB 32. The back drilled hole 40 removes a portion of the annular ring 44 at the back side surface of the via 34 as well as a corresponding portion of the via side wall plating 38 in the via 34 thereby disconnecting the remaining via side wall plating 38 from the heat sink 42.
In some applications, after the back drilling is performed the via can be filled from the backside, such as with soldermask or epoxy. FIG. 4 illustrates the PCB of FIG. 3 with the back drilled hole 40 filled with soldermask 54. This application is typically used if the contact pad, annular ring 44, on the front side (bottom surface in FIG. 4) is a solder pad where solder is applied. The soldermask within the via prevents the solder from extending through the via to the copper heat sink. FIG. 5 illustrates the PCB of FIG. 3 with the back drilled hole 40 and the remainder of the via 34 completely filled with epoxy 56. Additionally, a copper cap 58 is applied (plated) on the front side (bottom surface in FIG. 5) over the epoxy filled via. The copper cap 58 and annular ring 44 function as a solder pad.
Although effective in disconnecting the plated via from an attached heat sink, back drilling is time consuming and expensive. As via densities increase, such problems with back drilling become more acute.